Image forming apparatuses, which employ an electrophotographic method, create an image by fixing a toner image on a recording medium. Examples of the image forming apparatuses are a laser printer, a Light Emitting Diode (LED) printer and a digital photocopying machine.
In the electrophotographic method, an electrostatic latent image, which is in accordance with image information, is visualized by the toner (a developer), thereby creating a visible image. The visible image (a toner image) is transcribed and fixed on the recording medium. The electrostatic latent image is produced by electrifying the entire surface of a photoreceptor, then by irradiating the surface with light in accordance with the image information by use of a laser beam or LED. The visible image is created by the visualization of the electrostatic latent image with the toner (the developer) by a developing section. The fixation of the visible image onto the recording medium is carried out at a fixation section by fixing on the recording medium the visible image of the toner which was transferred onto the recording medium at a transfer section.
There has been greater demand for more compact image forming apparatuses, recently. In an image forming apparatus of the electrophotographic method, a toner storing section is targeted for size reduction to achieve miniaturization of the image forming apparatus because its occupying space is significantly large in the image forming apparatus. A large quantity of the toner must be stored in the image forming apparatus for user's convenience since the image forming apparatus may be used by more than one person and with a great number of printout, especially in the recent network environment.
Demand for color image output also has been increased recently. A color image forming apparatus, in which toners of three or four colors are used, needs a much larger space for a toner storing section in the image forming apparatus. Moreover, a bulky fixation section is necessary in case of a color image because the color is expressed by multi-color overlapping with a greater consumption of the toner on the recording medium such as paper or an Over Head Projection (OHP) sheet, thus requiring a greater application of heat for the thermal fixation, compared to the case of a monochrome image.
In addition, there is demand for a further energy-saving and environment-friendly method for manufacturing the toner. Today's common manufacturing methods of the toner are: (a) a method that involves melting, kneading, and grinding processes (MKG method), which has been employed conventionally, and (b) polymerization methods in a liquid solvent, which has been introduced recently. Known as the polymerization methods are, for example, suspension polymerization, emulsion polymerization, and dispersion polymerization methods.
Generally, the quantity of a coloring agent (carbon black or color pigments) contained in the toner is ranging from a few % up to about 10% by weight. The quantity of the toner necessary for achieving necessary image-intensity is between about 0.7 mg/cm2 and 1 mg/cm2. Due to those requirements, the image forming apparatus should store a large quantity of the toner, as described above.
Therefore, the quantity of the toner necessary for expressing the image information can be reduced by increasing the coloring agent content in the toner, thereby resulting in a smaller space occupied by the toner storing section in the image forming apparatus.
However, poor dispersibility of the coloring agent in the toner is often caused in the conventional toner and the conventional methods thereof when the coloring agent content in the toner is increased for reducing the quantity of toner used. With the poor dispersibility, the increase in the coloring agent content reduces the coloring power of the coloring agent, on the contrary.
Moreover, it is very difficult to further improve the dispersibility of the coloring agent in the polymerization methods. For example, the suspension polymerization method, which is the most popular among the polymerization methods, has difficulty in increasing the coloring agent content further than the current level with satisfactory dispersibility maintained. It is because the re-agglomeration of coloring agent particles tends to occur during the polymerization reaction with the increase in the quantity of the coloring agent, besides the problem in uniform dispersion of the raw materials (a mixture of monomers or coloring agents).
In addition, with respect to the dispersion of the coloring agent, the MKG method has an advantage over the polymerization methods that the large shear force is large in the melting and the kneading processes and the re-agglomeration of the coloring agent particles is prevented by a rapid cooling process following the kneading process.
However, for the toner produced by the MKG method, the way of manufacturing the toner, that is, prepararing a chip of a resin by melting and kneading, then grinding down the chip to targeted particle diameters, leads to susceptibility of the chip to cleavage at its resin part containing the coloring agent particles during the grinding process, so that the toner has a structure with a number of the coloring agent particles exposed from the surface of the toner. This has an adverse effect on the electric characteristics (charge characteristics) of the toner.
Moreover, the toner of the MKG method is mechanically weak at the interface between the coloring agent particles and the binding resin when a large quantity of the coloring agent particles is mixed in, so that stabile production of the toner with targeted particle-size distribution cannot be achieved due to the damage on the particles during the grinding process.
Further considering effects on the environment, the polymerization methods require some environmental measures such as washing and waste-fluid treatments for their organic solvents used in a large quantity. Furthermore, because the toner is produced in a liquid, the polymerization methods need a drying process that consumes a huge quantity of energy.